1. Field of the Invention
The present invention relates to a developing device and an image forming apparatus for image formation with a two-component developer including a toner and a magnetic carrier.
2. Description of the Related Art
Conventionally, electrophotographic image forming apparatuses such as copying machines, printers and facsimile machines have been known. In an electrophotographic image forming apparatus, an electrostatic latent image is formed on a surface of a photoconductor, a toner is supplied to the electrostatic latent image by means of a developing device to develop the electrostatic latent image, a toner image formed through the development is transferred and fixed onto a sheet such as a paper sheet.
In recent years, for a full-color compliant image forming apparatus and a high-definition compliant image forming apparatus, a developer (hereinafter, also referred to as a “two-component developer”) is often used which is excellent in toner charging stability. The two-component developer includes a toner and a carrier. The toner and the carrier are agitated in a developing device to generate friction between the toner and the carrier, and the friction allows the toner to be appropriately charged. The charged toner is supplied to a surface of a two-component developer holder (developing roller). The toner on the two-component developer holder is moved, by electrostatic attraction, to an electrostatic latent image formed on an image carrier (photoconductor). Thus, a toner image is formed on the photoconductor.
Since image forming apparatuses are required to be more high-speed and downsized, it is necessary to quickly perform sufficient charging of a two-component developer and to quickly convey the two-component developer.
In conventional image forming apparatuses, therefore, a circulative developing device in which a two-component developer is conveyed in a circulating manner has been employed in order to instantly disperse a supplementary toner into the two-component developer to give an appropriate amount of charge.
The circulative developing device, as disclosed in Japanese Unexamined Patent Application Publication No. 2001-255723, has two conveyance paths and two conveyance screws in a developer case, in which the conveyance paths allow a two-component developer to be conveyed and circulated therethrough, and the conveyance screws are respectively disposed in the conveyance paths to simultaneously agitate and convey the two-component developer. When the toner concentration of the two-component developer in the developing device falls below a predetermined value, toner is supplied to one conveyance path by a toner supply screw, and the toner supplied and the two-component developer are conveyed while being agitated.
In addition, Japanese Unexamined Patent Application Publication No. 2009-109741 proposes a developing device including, in a housing for containing a developer, a first agitation and conveyance screw, a second agitation and conveyance screw, and a developing roller, wherein the first and second agitation and conveyance screws each have a helical blade and a reverse helical blade having a phase (pitch) moving reversely to the helical blade, and the reverse helical blade is disposed at an downstream side end of the screw in the developer conveyance direction and has a through hole.
In this developing device, each reverse helical blade reversely conveys the developer and returns the developer through the through hole to generate convection in the developer near the reverse helical blade, so that the developer in the dead space is agitated efficiently and generation of a pool of the developer is prevented.
In such a conventional developing device, an auger screw having a helical blade is used as a conveyance member. However, the rigidity of the rotation axis of the auger screw is easily reduced when the rotation axis is formed thin or the rotation axis is formed of resin.
The reduced rigidity causes the auger screw to bend and therefore, in some cases, the helical blade is brought into contact with an inner wall of a developer tank. In such cases, toner stuck between the auger screw and the inner wall is aggregated to cause image defects such as insufficient image transfer.